Philip Jose Farmer passed away last week from natural causes. He was 91. For those that are true science fiction fans are familiar with his work.
My favorite has to be the Riverworld series. I read them as a kid and it has lasted with me all of these years. Riverworld is classified as straight science fiction, but it strikes me as being more in line with the predecessor of alternative history, in which famous people or events have a twist to it. This genre is one of my favorites and the most prolific author of this type is probably Harry Turtledove.
But to me, Farmer will be one of the earliest, and one of the best.